1. Field of the Invention
The present invention generally relates to the field of handling and interconnecting metal bumped integrated circuit chips, and more specifically to a method and apparatus for providing protective stops on the chips to protect the bumps during handling and to provide mechanical support to interconnected chips.
2. Description of the Related Art
To construct semiconductor devices, the chips must be handled without damaging them, and interconnected with sufficient mechanical support. In flip-chip applications semiconductor chips can be simultaneously electrically and mechanically interconnected using "cold welding" technology. The soft metal bumps, typically indium, on a chip are smashed together with the soft metal bumps on another chip, an integrated circuit board or a substrate to provide contact between the two devices.
In a flip-chip application for infrared imaging, a typical silicon readout chip includes a 644 by 480 array of transistors, each transistor has an indium bump for providing electrical contact to a detector chip. The detector chip includes an array of infrared sensitive diodes similar to the readout array, and each diode has an indium bump for contacting the readout chip. The detector chip is flipped over and interconnected to the readout chip by smashing or "cold welding" the indium bumps together to form a single contact connecting the two chips.
The diodes in the detector chip respond to the intensity of the infrared light incident on the silicon substrate and establish a potential voltage at each diode. The transistors in the readout chip output a current proportional to the potential across the respective indium contact. The indium bumps are fragile and easily damaged; the detector chips are the most susceptible to damage in standard handling.
FIGS. 1-3 illustrate a known indium bumped detector chip, and a known process for interconnecting a detector chip and a readout chip. In the sectional view shown in FIG. 1, a detector chip 10 includes an array of indium bumps 12 in electrical contact with respective diodes 13 disposed on the face of the chip. The soft metal indium bumps are exposed and fragile, and susceptible to damage during handling. As shown in FIG. 2, the detector chip is "flipped" and aligned with a readout chip 14 having an array of indium bumps 16 in electrical contact with respective transistors 15 formed on the face of the chip. Chip 10 is pressed down onto chip 14, smashing the pairs of indium bumps 12 and 16 into an array of contacts 18 connecting chip 10 to chip 14. FIG. 3 shows the interconnected chips.
The same properties of indium that facilitate cold welding for flip-chip applications also present significant problems. The metal is soft and weak, and the exposed bumps, as shown in FIG. 1, are easily damaged. These chips can not be handled with normal automated equipment. Therefore, indium bumped chips are typically handled manually, which is very slow and inefficient.
Furthermore, indium lacks sufficient strength to support the upper chip during normal handling. Currently, this problem is addressed by "wicking" the chips. This involves applying a drop of epoxy in between the chips. Normal plastics, such as epoxies, are approximately five times stronger than indium. The epoxy flows through the chips and encapsulates the indium contacts. This method provides sufficient mechanical support, but the epoxy is slow to process and adds capacitance to the electrical connection between the chips.
The capacitance is proportional to the dielectric constant of the medium between the two chips. Since the dielectric for epoxy is approximately 3.5 times that of air, encapsulating the indium contacts in epoxy increases the capacitance significantly. The additional capacitive loading increases power dissipation and decreases the speed of the chips. Quite often bubbles are formed in the epoxy, creating variable capacitances and different speeds within a circuit. For high frequency applications, the fragile unwicked chips or a different approach such as solder bumps must be used.
The interconnected chips are often encapsulated in plastic to prevent moisture from gathering on the chips. A barrier, such as epoxy, is needed to protect the bumps from the plastic encapsulation. Otherwise the encapsulation process will tear apart the chips. In these cases, the degraded electrical properties must be tolerated.
Because of the fragility of indium bumped chips, solder bumps are frequently used for general applications. A semiconductor chip with solder bumps is placed on top of another chip or substrate, and the solder is heated to form an interconnection. Solder has five times the tensile strength of indium, and is less susceptible to handling damage. However, solder has about one-third the creep strength of indium. Creep strength relates to a metals long term strength. Solder can withstand higher instantaneous loads but is very weak in terms of long-term constant loads. This creates reliability problems, and limits the overall size of solder connected chips. Additionally, solder work hardens as a result of thermal cycling and will break, whereas indium re-anneals at room temperature, and thus has a very long fatigue life. Furthermore, the spacing required between solder bump connections is far greater than for indium bumps.